Rheological correlations for oil-based drilling foams Testi Sherif a , Ramadan Ahmed a, * , Subhash Shah a , Mahmood Amani b a University of Oklahoma, 100 E. Boyd St, Norman, OK, USA b Texas A&M Engineering Building, Education City, PO Box 23874, Doha, Qatar article info Article history: Received 15 July 2016 Received in revised form 26 September 2016 Accepted 27 September 2016 Available online 28 September 2016 Keywords: Oil-based foam Viscosity Correlations Power law model Consistency index Fluid behavior index abstract This paper summarizes results of experimental investigation conducted on rheology of oil-based foams. Flow loop experiments were conducted varying base-liquid viscosity (3.4, 8.3, and 12.3 cP), flow rate (1 e52 L/min) and foam quality (34e68 percent). High-quality oil-based foams (greater than 68%) were not stable to perform rheology tests. The base-liquids were mixtures of mineral oil, diesel oil, and fluo- rosurfactant. A flow loop has designed and built to generate and circulate foam fluids to study their rheological behavior. Experiments were conducted at 690 kPa and ambient temperature (26 ± 2  C). To identify the existence of wall-slip, tests were conducted using different diameter (13.4,19.6 and 31.8 mm ID) pipe viscometers. Fully transparent PVC pipes were used as viscometers to visualize homogeneity of the foam. Foam degradation during the test was minimized by regenerating the fluid at the maximum flow rate for each flow measurement. Experimental results indicate expected foam viscosity trend that shows substantial viscosity increase with foam quality and base liquid viscosity. The foams displayed non-Newtonian (i.e. shear thinning) behavior, which improves with foam quality. For most field applicable shear rate range (10e1000 1/s), power law model best fits rheology of the foams. Like aqueous foams, consistency index increased with foam quality while fluid behavior index demonstrated moderate reduction. Slight right shifting of flow curve was observed in the small diameter viscometer. Since the shifting was not observed in other pipes, it can be attributed to wall-slip or experimental artifacts that can mimic a similar effect. Applying nonlinear regression analysis, an empirical model is formulated to compute power law parameters (n and K) of the foams. The data from small diameter pipe is not considered in the regression analysis due to the shifting of flow curves. Currently, existing models developed for aqueous and polymer based foams are often used for estimating flow behavior of oil based foams. However, discrepancies of the models can be very high when they are used for unintended applications. Hence, predictions of the new and existing models are rigorously compared. Results indicate that the new model has better accuracy than the existing ones. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Industrial application of foam is highly diversified. In the pe- troleum industry, it is used as displacing fluid in enhanced oil re- covery operation, fracturing fluid in stimulation process or wellbore circulating fluid in drilling operation to transport rock cuttings and control wellbore pressure. Foam is suitable for these applications because of its high viscosity, low density and low liquid content. Currently, new drilling techniques such as underbalanced and managed pressure drilling are being introduced to drill uncon- ventional wells. Drilling in depleted and low-pressure formations is very challenging with conventional method. Underbalanced dril- ling is often suitable for this type of formations. Stable foam with high-viscosity and low-density can provide the desired perfor- mance to carry out a successful underbalanced drilling operation. The existence of water-sensitive formation adds to the complexity of drilling a well with traditional aqueous based foams. Formation damage can occur in water-sensitive formation because of tempo- rary overbalance, chemical interaction or spontaneous imbibition. The problems can be mitigated using Oil-Based Foam (OBF). Foam is often described as a thermodynamically unstable mixture of liquid, gas and surfactant. Rheologically, foam is different from its constituents. It exhibits complex flow behavior, * Corresponding author. E-mail address: r.ahmed@ou.edu (R. Ahmed). Contents lists available at ScienceDirect Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse http://dx.doi.org/10.1016/j.jngse.2016.09.064 1875-5100/© 2016 Elsevier B.V. All rights reserved. Journal of Natural Gas Science and Engineering 35 (2016) 1249e1260